A typical radio wave antenna essentially performs two functions simultaneously or alternately, and in a transceiver system, the two functions may be achieved by the same antenna or by separate antennas. That is, as a transmitting antenna, an antenna generates a radio wave by radiating into the atmosphere an electrical signal produced by an electronic circuit, and as a receiving antenna, an antenna receives from the atmosphere such a radio wave for conversion into an electrical signal by a receiver circuit. For example, an antenna for receiving a television broadcast converts a broadcast radio wave into a current signal, which is then input to a television receiver.
Assuming that antenna characteristics, such as directional properties and operating frequency, are appropriate for a transmission (radiation) of radio waves, the reception of such radio waves by the same antenna follow the same characteristics. Hence, for the sake of convenience in explaining antenna operational principles and overall antenna theory, a transmitter antenna is frequently presumed to be a receiver antenna as well, such that antenna components such as radiating elements and feed lines are generally termed based on a transmitting operation. That is, an electrical signal output from a transmitter circuit is supplied to a feed line, which in turn supplies the signal to a radiating element, so that the radiating element can radiate a radio wave signal. A reception of radio waves by the same antenna follows a reverse sequence of the above steps, whereby the radiating element receives a radio wave from the atmosphere to produce an electrical signal, which is fed out through the feed line for input to a receiver circuit.
A good antenna has efficient transfer of energy properties and is tuned such that a large current signal is produced (peaked) for a given radio wave. While such criteria are applicable to both analog and digital broadcasts, their reception characteristics of a radio wave signal differ. Namely, reception quality continues to be degraded as received signal strength weakens in an analog television receiver, whereby ghosting occurs and the video display becomes increasingly snowy, while a similar reception by a digital television receiver maintains a high quality output until the received signal strength drops to a predetermined level, whereupon the reception quality is greatly degraded to include audio dropout and video blocking. Normal broadcast reception is enabled, in either system, by securing a proper level of received signal strength and signal-to-noise ratio and by overcoming multi-path problems.
A broadband slot antenna is applicable to the transmission and reception of television and AM/FM radio broadcasts, which use relatively low frequencies, i.e., long wavelengths, and therefore require relatively large antennas. Meanwhile, characteristics of an antenna for use by a broadcast station can be freely determined with little regard to cost or size, but in designing an antenna for receiving a broadcast signal, costs must be kept low and size should be minimized while maintaining required performance levels.
A slot antenna has long been in practical use due to its planar structure and its facilitation of broadband communications. A slot array antenna, made up of an array of hundreds or thousands of slots, has shortwave radar and satellite broadcast applications. The array of such an antenna enhances gain but increases overall antenna size, which inhibits application in VHF to low UHF broadcasting.
The slot antenna basically consists of one or more slots having a length of one half wavelength (λ/2). Power is fed to a slot using a microstrip (power feed line) intersecting the slot perpendicularly, the intersection usually occurring at the slot's center, for an efficient transfer of current (cross coupling) via a conductive connection, i.e., a short, or a capacitive coupling, i.e., an open, at a cross coupling point of the power feed line. To increase bandwidth in a slot antenna, the slot width is increased symmetrically from the slot's center to either end, to provide a variety of slot configurations, including bowtie, dog-bone, and paddle-bowtie configurations. Since the end width of any of these slots is considerably greater than that of a general slot, the resulting antenna size is increased accordingly. That is, when two such antennas are arranged in parallel, i.e., side by side, the space between the antennas (spatial margin) is increased over that of a normal slot antenna array, particularly in terms of width. Therefore, in arraying a plurality of such antennas, the accumulative space greatly increases antenna size. Moreover, if the spatial margin is too narrow, the antenna's electrical characteristics suffer.
General terminology for slot antenna technology, including “radiation,” “conductor,” “dielectric,” “radiating element,” “conductor plate,” “bowtie/dog-bone slot,” “dielectric substrate,” “microstrip antenna,” “array antenna,” “feed line,” and “coplanar waveguide,” is herein preferentially defined as follows.
Radiation is a phenomenon whereby radio waves are propagated from an antenna element into an empty space, i.e., the atmosphere.
A conductor is a material capable of carrying an electric current, and current tends to flow well in conductors made of molecularly dense materials, specifically metals such as silver, copper, gold, and aluminum. The radiating element of an antenna is essentially a conductor and, considering antenna cost and other factors, is primarily made of copper and/or aluminum. Silver and gold, in limited quantities, may also be used as the material for the radiating element.
A dielectric is a material which promotes electromagnetism but which inhibits a direct flow of an electric current and is therefore also termed an insulator. Due to these properties, a dielectric is generally used for creating a spatial separation between conductors of the radiating element of an antenna, while providing mechanical support for the conductors. Air can be considered a dielectric.
A radiating element is a structural component of an antenna and consists mainly of a conductor formed to radiate radio waves into the atmosphere. The radiating elements of a microstrip antenna may be a radiating patch or a radiating aperture. The radiating patch forms a radiating element using a conductor plate of a regular shape such as a circle, oval, triangle, quadrangle, or pentagon, while the radiating aperture forms a radiating element using a conductor plate perforated with an aperture of such a shape. When the aperture shape is a slot, the radiating aperture may be termed a radiating slot.
A conductor plate is a thin, flat plate made of a conductor and can be variously shaped to construct an antenna. One portion of a conductor plate (i.e., a radiating patch) may be used as a radiating element, while another portion of a conductor plate (i.e., a power feeding patch) may be used as a power feeding element. The radiating element may be a radiating slot, for example, a bowtie slot or dog-bone slot, which is formed by removing a portion of a conductor plate. A feed line may be formed by removing most of a conductor plate, leaving only a strip of the conductor material. A radiating patch and a feed line may be simultaneously formed by removing predetermined portions of a conductor plate. A conductor plate can also be used as a grounded conductor plane and/or a reflective plate.
A bowtie or dog-bone slot extends the operational bandwidth of a slot antenna. That is, while a slot antenna's operational bandwidth is relatively narrow when a conductor plate has a plurality of uniform slots, the formation of a bowtie slot or dog-bone slot will extend its operational bandwidth. A bowtie slot configuration is achieved by a radiating slot having a symmetrically increasing slot width toward the slot ends, and a dog-bone slot configuration is achieved by circular radiating apertures communicating with the slot ends. A slot antenna may have features of both types, i.e., a bowtie slot and a dog-bone slot.
A dielectric substrate is a thin, flat plate made of a dielectric (insulator). In configuring an antenna, the dielectric substrate is used for isolating a pair of conductor plates, to thereby be separated by a uniform distance. A conductor plate, having a surface area corresponding to the area of the dielectric substrate, can be abutted against one or both faces of a dielectric substrate, as in printed circuit board (PCB) technology, and similar to PCB technology, a dielectric substrate is called a single-face dielectric substrate when the conductor plate is abutted against one face and is called a double-face dielectric substrate when the conductor plate abuts both faces. Accordingly, a broadband slot array antenna may use a single-face dielectric substrate, a double-face dielectric substrate, and/or a simple dielectric substrate (i.e., without any conductor plate), to produce a stacked structure (i.e., without adhesion) where a dielectric substrate is alternately stacked on a conductor plate or a conductor plate is alternately stacked on a conductor plate. Thus, in addition to a single-face dielectric substrate, a triple-decked (sandwiched) structure may be provided, whereby one dielectric substrate is disposed between a pair of conductor plates.
A microstrip antenna is essentially a panel-type (flat) antenna fabricated by forming radiating elements or feed lines on a conductor plate used with one or more of the various dielectric substrates. Hence, the microstrip antenna has a structure in which conductor plates and dielectric substrates are alternately stacked. For instance, a general microstrip antenna has a five-layer structure, stacking a grounded conductor layer (a first layer), a dielectric layer (a second layer), a feed line conductor layer (a third layer), a dielectric layer (a fourth layer), and a radiating element conductor layer (a fifth layer). The fabrication of such a structure can be provided by interposing dielectric layers between the first, third, and fifth layers each having a predetermined shape or by simultaneously processing (shaping) the first, second, and third layers on one double-face dielectric substrate, simultaneously processing the fourth and fifth layers on one single-face dielectric substrate, and then stacking the two dielectric substrates. The feed lines or radiating elements can be fabricated from conductor plate material attached to a single-face or double-face dielectric substrate in a manner very similar to that of fabricating a printed circuit board, resulting in an antenna called a PCB antenna, whereby a desired portion of the conductor plate material is defined using photolithography and the remainder is chemically removed by etching. Regardless of the method of fabrication, however, a microstrip antenna is generally called a PCB antenna.
An array antenna consists of at least two radiating elements arranged into a parallel array. Typically, however, the radiating elements number in the tens to hundreds and may even number in the tens of thousands. The radiating elements may be radiating patches or radiating slots.
A feed line is a line (conductor) for supplying an electrical signal to a radiating element, which may be achieved by a conductive connection or a capacitive coupling. The feed line mainly uses a microstrip but may use a coaxial line, a coplanar waveguide, a slot line, or the like. A power feed line uses a power-feeding element.
A coplanar waveguide is a planar transmission line created by forming (e.g., etching) in a conductor plate a corresponding pair of parallel slots, to leave a strip of conductor plate material between the slots. Preferably, the slots have the same width.
FIGS. 1A–1K show general slot antenna configurations, in which a slot is formed in one of several conductor plates 100a–100k and a pair of feed lines 112 are respectively connected across the center of the slot.
In FIG. 1A, a basic slot 101 has a simple narrow slot formed in the conductor plate 100a and exhibits a narrow operational bandwidth, which can be increased by modifying this basic configuration. For instance, a bowtie slot 102 or 103 is a modification of the basic slot antenna, in which broadband characteristics are achieved by increasing the slot width symmetrically toward the slot ends, whereby the slot is formed by straight slot sides as in FIG. 1B or by curved slot sides following the locus of an exponential function as in FIG. 1C. A wide slot width may be kept constant over the length of each symmetrical half of a bowtie slot, forming the paddle shape of a paddle-bowtie slot 104, as shown in FIG. 1D.
Broadband characteristics may also be achieved by forming an aperture communicating with each end of the basic slot 101 to provide a dog-bone slot 105 (FIG. 1E), a T-type slot 106 (FIG. 1F), or a Y-type slot 107 (FIG. 1G). Further antenna configurations include antenna slots 108 (FIG. 1H), 109 (FIG. 1I), 110 (FIG. 1J), and 111 (FIG. 1K) by forming rectangular, triangular, fan-shaped, or semicircular apertures communicating with the basic slot 101.
Referring to FIG. 2, illustrating the configuration of a broadband paddle-bowtie slot antenna according to a related art, a pair of wide slots 206 respectively communicate with each side of a bowtie slot 205, thereby forming a paddle-bowtie slot 231 having a paddle shape. The antenna is driven by feeding power to the slot via a microstrip 218 opposing a conductor plate 200, which is essentially a grounded surface. Thus, the microstrip 218 is a feed line extending from an input terminal 216 to a contact terminal 210, which traverses a slot neck 201 to be connected (grounded) to the conductor plate 200. The feed line may be constituted by a modified microstrip, a coaxial line, coplanar waveguide, or slot line.
To increase operational bandwidth, a plurality of slots are arrayed to form a broadband slot array antenna. Representative of such an array, FIG. 3 shows a paddle-bowtie slot array antenna according to a related art, in which broadband paddle-bowtie antennas 306 and 307 are arranged in parallel together on a conductor plate 300. To reduce a mutual interference occurring between the two antennas, a minimum inter-antenna element distance d should be maintained, where d is equal to at least λ/2. This minimum distance proscribes further miniaturization of a broadband paddle-bowtie slot array antenna, which limits its application.